Metal ions are essential to life as they augment amino acid protein chemistry and thereby catalyze many difficult biological reactions. As ?free? metal ions are toxic and indiscriminately reactive, critical protein systems have evolved to sequester, chaperone, and regulate metal ion concentrations. Defects in these systems lead to metal ion metabolic disease and result in cellular, tissue, and systemic pathology. The iron-sulfur cluster assembly pathway contains a conserved set of proteins that synthesizes Fe-S clusters, which are then distributed by a network of cluster transfer and conversion factors to hundreds of Fe-S dependent proteins. Here we focus on the structure of the eukaryotic Fe-S assembly complex and the mechanism of cluster biosynthesis. We show that a stable low activity Fe-S assembly complex can be activated by binding of the Friedreich's ataxia protein (frataxin; FXN), and that the mitochondrial acyl carrier protein (ACP) has a moonlighting function as a component critical for the stability and function of the assembly complex. Further, our crystal structure of the core (NFS1-ISD11-ACP; called SDA) of the eukaryotic Fe-S assembly complex revealed a dramatically different architecture compared to the prokaryotic system that is stabilized by a novel ACP-lipid interaction with the hydrophobic core of ISD11. Here, we will build upon these paradigm shifting results to determine structural and dynamic information for binding accessory proteins to the SDA core of the Fe-S assembly complex, elucidate how ACP-lipid interactions modulate functional properties for the assembly complex, and provide mechanistic insight into FXN based activation and Fe-S cluster formation. This fundamental research will establish a framework for emerging genetic results and discoveries and provide a basis for understanding defects in iron-sulfur cluster metabolism relevant to human health and disease.